Evolved Packet System (EPS) is the Evolved 3GPP Packet Switched Domain and includes Evolved Packet Core (EPC) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
FIG. 1 is an overview of the EPC architecture. This architecture is defined in 3GPP TS 23.401, which provides definitions of the PGW (PDN Gateway), SGW (Serving Gateway), PCRF (Policy and Charging Rules Function), MME (Mobility Management Entity), and mobile device (UE). The LTE radio access, E-UTRAN, includes one more eNBs (also referred to as base stations). FIG. 1 illustrates non-roaming EPC architecture for 3GPP accesses.
FIG. 2 shows an overall E-UTRAN architecture and is further defined, for example, in 3GPP TS 36.300. The E-UTRAN includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC in addition to the user plane protocols) protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME interface and to the Serving Gateway (S-GW) by means of the S1-U interface.
Portions of the EPC Control Plane (CP) and User Plane (UP) architectures are shown in FIGS. 3 and 4. FIG. 3 illustrates the EPC Control Plane protocol architecture, and FIG. 4 illustrates the EPC User Plane protocol architecture.
LTE Dual Connectivity is a solution standardized in 3GPP Rel-12 to support UEs connecting to multiple carriers to send receive data on multiple carriers at the same time. Below is an overview description and more details can be found in 3GPP TS 36.300.
E-UTRAN supports Dual Connectivity (DC) operation whereby a multiple Rx/Tx UE in RRC_CONNECTED is configured to utilize radio resources provided by two distinct schedulers, located in two eNBs connected via a non-ideal backhaul over the X2 interface (see TR 36.842 and TR 36.932). The overall E-UTRAN architecture as specified in previous section and depicted in FIG. 2 is applicable for DC as well. eNBs involved in DC for a certain UE may assume two different roles: an eNB may either act as an MeNB (Master eNB) or as an SeNB (Secondary eNB). In DC, a UE is connected to one MeNB and one SeNB.
In DC, the radio protocol architecture that a particular bearer uses depends on how the bearer is setup. Three bearer types exist: MCG (Master Cell Group) bearer, SCG (Secondary Cell Group) bearer and split bearer. Those three bearer types are illustrated in FIG. 5 which illustrates Radio Protocol Architecture for Dual Connectivity. RRC is located in MeNB and SRBs (Signaling Radio Bearers) are always configured as MCG bearer type and therefore only use the radio resources of the MeNB. Note that DC can also be described as having at least one bearer configured to use radio resources provided by the SeNB.
Inter-eNB control plane signaling for DC is performed by means of X2 interface signaling. Control plane signaling towards the MME is performed by means of S1 interface signaling.
There is only one S1-MME connection per DC UE between the MeNB and the MME. Each eNB should be able to handle UEs independently, i.e. provide the PCell to some UEs while providing SCell(s) for SCG to others. Each eNB involved in DC for a certain UE controls its radio resources and is primarily responsible for allocating radio resources of its cells. Respective coordination between MeNB and SeNB is performed by means of X2 interface signaling.
FIG. 6 shows C-plane connectivity of eNBs involved in DC for a certain UE: the S1-MME is terminated in MeNB, and the MeNB and the SeNB are interconnected via X2-C. FIG. 6 illustrates C-Plane connectivity of eNBs involved in Dual Connectivity
For dual connectivity, two different user plane architectures are allowed: one in which the S1-U only terminates in the MeNB and the user plane data is transferred from MeNB to SeNB using the X2-U, and a second architecture where the S1-U can terminate in the SeNB. FIG. 7 illustrates different U-plane connectivity options of eNBs involved in DC for a certain UE. Different bearer options can be configured with different user plane architectures. U-plane connectivity depends on the bearer option configured:                For MCG bearers, the S1-U connection for the corresponding bearer(s) to the S-GW is terminated in the MeNB. The SeNB is not involved in the transport of user plane data for this type of bearer(s) over the Uu.        For split bearers, the S1-U connection to the S-GW is terminated in the MeNB. PDCP data is transferred between the MeNB and the SeNB via X2-U. The SeNB and MeNB are involved in transmitting data of this bearer type over the Uu.        For SCG bearers, the SeNB is directly connected with the S-GW via S1-U. The MeNB is not involved in the transport of user plane data for this type of bearer(s) over the Uu.        
FIG. 7 illustrates U-Plane connectivity of eNBs involved in Dual Connectivity. Note that if only MCG and split bearers are configured, there is no S1-U termination in the SeNB.
The SeNB Addition procedure is initiated by the MeNB and is used to establish a UE context at the SeNB in order to provide radio resources from the SeNB to the UE. This procedure is used to add at least the first cell (PSCell) of the SCG. FIG. 8 shows the SeNB Addition procedure.
Due to variable radio channel quality between an eNB and a UE, a temporary interruption in data traffic may occur. Accordingly, there continues to exist a demand for methods to accommodate such temporary interruptions.